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LHRH Receptor, LH and FSH Concentrations in Anterior Pituitaries of Cycling, Noncycling and Early Pregnant Heifers^1,2,

Washington State University^,5, Pullman 99164-6332

Abstract

In domestic animals limited data are available concerning levels of pituitary luteinizing hormone-releasing hormone (LHRH) receptors during various physiological states. The objectives of this study were to quantify anterior pituitary gonadotropin and LHRH receptor concentrations in cycling, noncycling and early pregnant beef heifers. To accomplish these objectives, five heifers each were slaughtered, after synchronization with prostaglandin F₂: (PGF₂), on d 0 (estrus), 7 and 14 of the estrous cycle and d 40 of pregnancy. Four heifers determined to be noncycling were also slaughtered. Pituitaries were collected and analyzed for LHRH receptor and gonadotropin concentrations. Pituitary luteinizing hormone (LH) concentrations were low on d 0 (1.4 ± .2 µg/mg pituitary, mean ± SE) and remained low on d 7 (1.4 ± .1 µg/mg pituitary) before increasing (P<.01) on d 14 (2.6 ± .5 µg/mg pituitary). Luteinizing hormone concentrations, compared with d 0, were also elevated (P<.01) in noncycling (NC; 2.6 ± .2 µg/mg pituitary) animals and in 40-d pregnant (PG; 2.5 ± .2 µg/mg pituitary) heifers. Pituitary follicle stimulating hormone (FSH) concentrations, though similar (P>.05) for all groups, paralleled changes in LH concentration. Pituitary LHRH receptor binding affinity did not differ (P>.05) among groups, with an overall K_d = .64 ± .02 x 10^–9 M. Luteinizing hormone-releasing hormone receptor concentrations were highest on d 0 (1.09 ± .12 fmol/mg pituitary) and fell (P<.01) to low levels on d 7 (.75 ± .11 fmol/mg pituitary). Luteinizing hormone-releasing hormone receptor values on d 14 (1.08 ± .08 fmol/mg pituitary) were similar (P>.05) to d 0 concentrations. Pituitaries of NC heifers had only 45% of the LHRH receptor concentration (P<.001; .50 ± .06 fmol/mg pituitary) of pituitaries from d 0 heifers, while LHRH receptor concentrations during early pregnancy (.81 ± .06 fmol/mg pituitary) were 70% (P<.05) of those during estrus. These results suggest that LHRH receptor concentrations in cattle are dynamically changing with physiological status and are probably, in part, responsible for changes in pituitary responsiveness. Specifically, LHRH receptor numbers are elevated at estrus when pituitary responsiveness has been reported to be the highest, while noncyclicity may be associated with depressed levels of pituitary LHRH receptors.

¹ Scientific Paper No. 6822, College of Agriculture and Home Economics Research Center, Washington State Univ., Project 0137.

² The authors wish to thank J. W. Paul and J. Sandow of Hoescht A. G. for the generous gift of [D-Ser-(TBU)⁶-des-Gly-NH₂¹⁰]-LHRH-ethylamide.

³ Present address: Dept. of Anim. Sci., Oklahoma State Univ., Stillwater, OK 74078.

⁴ To whom reprint requests should be addressed.

⁵ Dept. of Anim. Sci.